Silicoaluminophosphates (SAPOs) are synthetic materials having a three-dimensional microporous aluminophosphate crystalline framework with silicon incorporated into the framework. The framework structure consists of PO2+, AlO2−, and SiO2 tetrahedral units. The empirical chemical composition, on an anhydrous basis, is:mR:(Si.xAlyPz)O2 wherein, R represents at least one organic templating agent present in the intracrystalline pore system; m represents the moles of R present per mole of (SixAlyPz)O2 and has a value from zero to 0.3; and x, y, and z represent the mole fractions of silicon, aluminum, and phosphorous, respectively, present as tetrahedral oxides. These compounds were disclosed in U.S. Pat. No. 4,440,871 as being a class of silicon-substituted aluminophosphates, which are both crystalline and microporous and exhibit properties characteristic of both aluminosilicate zeolites and aluminophosphates. The general principles of zeolite synthesis are well documented, e.g. “Synthesis of High-silica Aluminosilicate Zeolites”, Study of Surface Science and Catalysis, vol. 33, Elsevier, 1987 and “Synthesis of Aluminosilicate Zeolites and Related Silica-based Materials”, Catalysis and Zeolites: Fundamentals and Applications, Springer, 1999.
U.S. Pat. Nos. 6,773,688 and 7,052,664 disclose a process for manufacturing a SAPO-34 silicoaluminophosphate molecular sieve, the process comprising the steps of: (a) providing sources of aluminum, of phosphorus and of silicon, wherein the source of silicon is in solution with a water-miscible organic base; (b) forming a synthesis mixture from the sources; and (c) treating the synthesis mixture for a period of time and at a temperature sufficient to form the silicoaluminophosphate molecular sieve. These patents also disclose:                In the manufacture of SAPO-34, it has proved advantageous to use a mixture of templates. For example, a suitable template mixture is TEAOH, which is readily soluble in water, and DPA, which dissolves in water with some difficulty.        
U.S. Pat. No. 7,459,136 discloses a process for manufacturing a silicoaluminophosphate molecular sieve, the process comprising the steps of: (a) dissolving a silicon source into a solution containing a structure-directing template at conditions sufficient to establish a dissolved silicon concentration of at least 0.05 wt. %; (b) adding at least one aluminum source and at least one phosphorus source to at least a portion of the solution of step (a) to form a synthesis mixture, wherein at least the major portion of the aluminum source and phosphorus source are added to the solution after the solution has reached a dissolved silicon concentration of at least 0.03 wt. %; and (c) treating the synthesis mixture at a temperature sufficient to form the silicoaluminophosphate molecular sieve, wherein the silicoaluminophosphate molecular sieve comprises at least one intergrown phase of molecular sieves having AEI and CHA framework types, wherein the intergrown phase has an AEI/CHA mass ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis.
U.S. Pat. No. 7,645,718 discloses a process for preparing Fe-exchanged SAPO-34 by a liquid phase ion-exchange method using an iron salt solution. Only small amounts of Fe (≦1.4%) were exchanged onto SAPO-34 using liquid phase ion exchange.
The process for preparing Fe-SAPO-34 by sublimation of FeCl3 for use in SCR applications is disclosed in Kucherov et al., Catalysis Letters 56 (1998) 173-181. The dispersion of Fe in SAPO-34 is not as good as on the medium pore ZSM-5 using the sublimation method.
U.S. Pat. No. 7,785,554 discloses a process for manufacturing a silicoaluminophosphate molecular sieve comprising at least one intergrown phase of AEI and CHA framework types, the process comprising the steps of: (a) combining at least one silicon source, at least one phosphorus source, at least one aluminum source, and at least one structure-directing-agent (R) to form a mixture; and (b) treating the mixture at crystallization conditions sufficient to form the silicoaluminophosphate molecular sieve, wherein the mixture prepared in step (a) has a molar composition of: (n)SiO2/Al2O3/(m)P2O5/(x)R/(y)H2O wherein n ranges from about 0.005 to about 0.6, m ranges from about 0.6 to about 1.2, x ranges from about 0.5 to about 0.99, and y ranges from about 10 to about 40.
U.S. Pat. No. 8,541,331 discloses a method for preparing iron-containing aluminosilicate zeolites having both framework iron and iron cations on ion-exchange sites.
U.S. Pat. No. 8,603,432 discloses a method for preparing Fe-SAPO-34 from a slurry of SAPO-34 in a ferric nitrate solution.
U.S. Patent Application Publication No. US 2012/0251422 A1 discloses an Fe-SAPO-34 molecular sieve, where the molecular sieve contains both framework iron and iron cations at ion-exchange sites. A method of making a catalyst comprising Fe-SAPO-34 comprises mixing sources of an iron salt, alumina, silica, phosphate, at least one organic structural directing agent and water to form a gel; heating the gel in an autoclave at a temperature ranging from 140 to 220° C. to form a crystalline Fe-SAPO-34 product; calcining the product; and contacting the product with acid or steam.
The literature also describes iron being loaded into SAPO-34 using different iron salts. (Y. Wei et al., Catalysis Today, 131, 2008, 262-269; M. Kang et al., Journal of Molecular Catalysis A: Chemical 160, 2000, 437-444; M. Kang et al., Envirn. Eng. Res, 3, 3, 175-182, 1998; and M. Heon et al., Korean Journal of materials research, 9, 8, 1999)
There is a need for an improved and simplified method of making Fe-SAPO-34 that does not require ion-exchange or impregnation, where the Fe-SAPO-34 displays good catalytic activity and stability. The Inventors have discovered a method of directly synthesizing Fe-SAPO-34 that contains both framework iron and iron cations at ion-exchange sites and various forms of the Fe-SAPO-34 produced by the methods.